EP 0 734 494 A1 discloses a supply device configured in this type. The piston is a component of a unit which may be handled independently. The unit additionally comprises a second non-return valve for the ventilation of the working chamber, and the resetting spring. This arrangement allows inserting the mentioned components as a unit into a bore in the accommodating member. In a separate step the carrier for the non-return valve is inserted into the extended bore, while the resetting spring is elastically preloaded at the same time. The process reliability of this mounting operation needs improvement because the resetting spring may buckle under increasing preload. To prevent this, special adaptation measures are provided. Those measures shall be avoided.